Progesterone and estrogen have opposing effects on vaginal physiology. Estrogen promotes vaginal epithelial maturation and decreases pH, while progesterone is an estrogen antagonist. Previously, we showed that systemic progesterone enhanced simian immunodeficiency virus (SIV) vaginal transmission in ovary-intact females. During this grant period we showed that systemic estrogen completely blocked SIV vaginal transmission in ovariectomized (ovx) macaques. We also found that SIV vaginal transmission was enhanced in untreated ovx macaques, providing a new model for HIV transmission in post-menopausal women. New results show that the effects of estrogen were confined to the vaginal microenvironment. To capitalize on these findings, we propose to test topical estrogen for prevention of SIV vaginal transmission. The application was extensively revised in response to the critique. The studies on humans have been deleted. In the revision, topical estrogen will be tested in both ovx and in cycling macaques. These experiments will model the post-menopausal and pre-menopausal states. The new toxicity studies on topical estrogen will use sensitive assays for leutenizing hormone (an indictor of systemic estrogen leakage) and for endometrial hyperplasia. The aims are; Modified Specific Aim 1. To test the safety and efficacy of topical estrogen on SIV-vaginal transmission in ovx macaques. New Aim 2. To test the safety and efficacy of low dose, vaginal estrogen in protecting cycling macaques against SIV vaginal transmission. Using the macaque model, the aims focus on a practical and safe application of our findings to sexually active women. Sexually active post-menopausal women are a growing risk group. Our studies could provide a method to reduce the risk to this population. The need for protecting pre-menopausal women is self-evident. This multi-disciplinary approach seeks to show prophylactixis in the SIV model that can be directly applied to HIV vaginal transmission.